1. Field of the Invention
This invention relates to a thermal printhead of the type which comprises a row of heating dots covered by a protective coating.
2. Description of the Related Art
As shown in FIG. 5 of the accompanying drawing, a typical prior art thermal printhead 1' comprises an insulating head substrate B' supported on a heat sink plate 2', and a connector circuit board 2' also supported on the heat sink plate. The head substrate 3' is formed with a wiring conductor pattern (not shown), whereas the connector circuit board 4' is also formed with a wiring conductor pattern (not shown) held in electrical contact with the wiring conductor pattern of the head substrate 3' by means of a metallic presser cover 5'.
The head substrate 3' further carries a resistor strip 7' and an array of drive ICs 6' for divisionally activating the resistor strip 7' to generate heat. The resistor strip 7' together with the unillustrated wiring conductor pattern on the head substrate 3' is covered by a protective glass coating (not shown).
In operation for printing, a thermosensitive paper 8' backed up by a platen 9' is held in sliding contact with the unillustrated glass coating at the resistor strip 7'. Thus, static electricity is inevitably generated by such sliding contact. However, since the presser cover member 5' is made of metals the generated static electricity is allowed to escape through the presser cover member 5'.
On the other hand, there is also known a thermal printhead which has no presser cover but is otherwise similar to the one shown in FIG. 5. Such a printhead is advantageous because of a size reduction. However, due to the absence of the metallic presser cover, static electricity frictionally generated at the resistor strip is abruptly discharged to the wiring conductor pattern when the static electricity is charged to a high level. As a result, the drive ICs may be electrostatically damaged.
For solving the above problem, it is conceivable to increase the thickness of the protective glass coating as a whole, thereby preventing an electrostatic discharge through the protective glass coating into the wiring conductor. However, this solution inevitably decreases heat transmission from the resistor strip to the thermosensitive paper, thereby resulting in a deterioration of printing quality.